1. Field of the Invention
The present invention relates to a keypad assembly, and more particularly to a keypad assembly in which a whole surface of the keypad is illuminated.
2. Background of the Related Art
Mobile terminals often include a backlighting function which allows buttons on a terminal keypad to be manipulated even under dark or poor lighting conditions. Light emitting diodes (LEDs) are generally used as the light sources for this keypad backlighting function.
FIG. 1 is a side-sectional view of a folder-type mobile terminal which employs a conventional technique for backlighting a keyboard, and FIG. 2 is an enlarged view of a portion ‘A’ of FIG. 1. As shown in these figures, the mobile terminal includes a main body 10 consisting of upper and lower cases 11 and 12 and a cover 20 connected to the main body 10 by a hinge mechanism 30. The cover includes a display unit 21 for displaying an operation state of the terminal and a message, and a receiving part 22 for outputting a voice.
The upper case 11 of the main body 10 is provided with a plurality of button holes, and a keypad 14 having an equal number of domes 14b fitted on a lower surface of upper case such that the domes 14b protrude through corresponding button holes of the upper case. Each dome 14b is formed to be upwardly raised such that a corresponding button 14c is filled in a lower cavity of the dome.
A PCB 15 is located below the keypad with a certain gap therebetween. The PCB is provided with a plurality of actuators 15a installed just below corresponding buttons 14 so as to generate an electric signal when each button 14 is pressed. Light emitting diodes 15b are arranged between the actuators 15a for backlighting the key pad 14.
FIG. 3 is a circuit diagram of an LED driving circuit of the conventional backlighting apparatus. In this driving circuit, a collector and an emitter of a switch transistor (Q1) are connected when a control voltage of 3.0V is applied to the base terminal, thereby causing the LEDs 15b to emit light. In the conventional backlighting apparatus, about 6 to 12 LEDs, each consuming about 60-120 mA, are used for backlighting the keypad 14.
FIG. 4 is a top plan view of the conventional keypad backlighting apparatus. In this figure, reference numeral 17 denotes an emitting coverage of LEDs 15b. As shown, even though 10 LEDs are disposed around the PCB surface below the keypad for illuminating keypad 14, the illumination coverage is locally limited around the respective LEDs so as not to evenly illuminate the entire keypad.
It is therefore clear that the conventional backlighting apparatus has significant drawbacks. First, in the conventional apparatus the illumination coverage differs depending on the installation positions of the LEDs, and these installation positions fail to allow the entire keypad to be evenly illuminated. Second, 6-12 LEDs are required to be installed on the PCB of the conventional apparatus. This complicates the terminal design and limits its ability to be compact. Third, installation of the LEDs around the buttons degrades the operational sensitivity of the LED actuators.
From the foregoing discussion, it is apparent that there is a need for an improved way of backlighting a keypad of a mobile terminal which does not realize any of the drawbacks of conventional methods.